Hopper rail cars are designed for the transport of dry products such as plastic resins, flour, sugar or other granular or powdered products. Such cars typically have one or more large manhole size hatches on the top and one or more smaller spouts on the bottom. These hatches and spouts are covered with metal covers and various sealing devices to be discussed later. The product to be transported is generally loaded through the top hatches such as from a hose or funnel at a bulk facility. The hatches are then closed and the car is transported to its unloading destination. At the unloading destination a vacuum hose is attached to one or more of the lower spouts and one or more of the upper hatches are opened to provide a vent. The product is then unloaded by applying a vacuum to the car through the lower spout with several lower spouts being used as required. Additional venting is frequently provided by opening lower spouts on the opposite side of the rail car from the lower spout to which the vacuum hose is attached. During shipment, in order to protect the material in the car from contamination, seals made from thin plastic membranes are usually placed over the top hatches and the lower spouts underneath the metal covers on these openings. When an opening is used for unloading, the plastic membrane seal must be removed to allow either the passage of product or the passage of air to provide a vent.
When the plastic seals are removed for unloading purposes, this leaves the interior of the car and its contents open to contamination from the surroundings. This contamination is accelerated when a vacuum is drawn on the unloading spout thereby pulling air from the atmosphere into the rail car through the vent openings. Along with the air pulled in, dirt and other contaminants are frequently pulled into the rail car interior thereby contaminating the product inside. In order to reduce the amount of this contamination, it is currently known to place portable filters over the openings being used as vents. These portable filters, however, are generally bulky and hard to handle; for that and other reasons, the use of the filters is frequently ignored by personnel responsible for unloading such hopper rail cars at the unloading facility. Even when the portable filters are used by unloading personnel, they do not generally fit tightly enough on the vent openings to provide a positive seal in order to exclude any contamination while allowing the required venting rate of air flow. A system and filter have been needed, therefore, to provide for positive filtration of the air entering the vent openings during unloading.